Immunosuppressive therapy can produce hematologic improvement in a large proportion of patients with severe aplastic anemia. We have established anti-thymocyte globulin and cyclosporin as the current treatment of choice for patients who do not have histocompatible sibling donors or who are otherwise ineligible for allogeneic bone marrow transplant. About 70% of patients respond to this therapy. We are currently following 112 patients and continuing to accrue patients onto this protocol to determine the durability of response, the incidence of late complications, and the long- term prognosis. Interim results have been published.